If this is a friend for your existing guinea pig, be sure to quarantine first. It is important that both your existing guinea pig(s) and your new guinea pig(s) are healthy.
http://www.guinealynx.info/quarantine.html

Clairey's Introduction Tips: A MUST READ! - Unusual ways to shake things up!
"When I intro'd Stephen and Robert, they had my kitchen floor for about 5 hours to make sure they were properly exhausted before going into the cage. I rebuilt the cage so it was different for both pigs. I sat at the edge of the kitchen on a stool armed with a dustpan, cutlery, plastic cups and a couple of towels. If things looked to be getting too intense, I'd drop a fork or cup on the floor to make a distracting noise. I didn't speak or look at them when I did it. The towels got thrown across the space onto a counter top (imitation house eagles). It was enough to make them pause at least or sometimes huddle together in a corner. Food periodically raining from the sky when they were calm helped too. I usually feed in a bowl or a heap but I didn't want any guarding so I scattered all over the place and let them find it. I kept that up for a few weeks after they moved in together too. Oh, and I used the fleece they'd stanked during the intro to line the cage for a few days to save them starting the whole palaver again. I do recall buddy bathing them at one point but I think that was after a few days together to seal the deal.

"Sights and sounds that startle them happen every day, squirts only happen when there's a human around so I'd be concerned about making them wary of humans. I don't even know if their minds work like that, mind you. That's why I don't look at them or speak when using a distraction. So to them it's not me being mean, it's just 'the big, scary world' doing what it does ever-so-coincidentally when they're facing off!" [Tips added by Lynx]

See also:

Cavy Spirit: IntroductionsCavy Spirit has the most excellent and informative page on introducing guinea pigs. All types of "social" behaviors are covered, including what is normal, and what is considered combative. ( www.CavySpirit.com )